Conversion of high molecular weight petroleum feeds to more valuable products by catalytic processes such as fluidized catalytic cracking is important to petroleum processes. In the fluidized catalytic cracking process, higher molecular weight feeds are contacted with fluidized catalyst particles in the riser reactor of the fluidized catalytic cracking unit. The contacting between feed and catalyst is controlled according to the type of product desired. In catalytic cracking of the feed, reactor conditions such as temperature and catalyst circulation rate are controlled to maximize the products desired and minimize the formation of less desirable products such as light gases and coke.
Miscellaneous fluidized catalytic cracking reactor riser and reactor vessel designs have been utilized in the past. However, with the advance of zeolitic cracking catalysts with greatly improved cracking activity, most modern fluidized catalytic cracking reactors utilize a short contact-time cracking configuration. With this configuration, the time in which the catalyst and the fluidized catalytic cracker feedstream are in contact is limited in order to minimize the amount of excessive cracking which results in the increased production of less valued products such as light hydrocarbon gases as well as increased coking deposition on the cracking catalysts. Short contact-time riser reactor designs are relatively new to the petrochemical industry, but have gained wide-spread acceptance and use in the industry due to the ability of optimizing hydrocarbon cracking products and yields in conjunction with the use of modem cracking catalysts. One such design for short contact-time fluid catalytic cracking reactor riser designs is illustrated in U.S. Pat. No. 5,190,650 to Tammera et al.
Most short-contact time fluidized catalytic cracking configurations utilize a reactor riser cracking configuration wherein the catalyst is contacted with the fluidized catalytic cracker feedstock in a reactor riser, and the catalyst and the hydrocarbon reaction products are separated shortly after the catalyst and hydrocarbon mixture leaves the reactor riser and enters the fluidized catalytic cracking reactor. Although there are many different fluidized catalytic cracking reactor designs in use, most use mechanical cyclones internal to the reactor to separate the catalyst from the hydrocarbon reactor products as quickly and efficiently as possible. This rapid separation process has the benefits of both minimizing post-riser reactions between the catalyst and the hydrocarbons as well as providing a physical means for separating the cracked hydrocarbon products for further processing from the spent catalyst which is regenerated prior to reintroduction of the regenerated catalyst back into the reaction process.
Significant improvements in catalyst technology have led to most conventional fluidized catalytic cracking reactors being designed for short contact-time processing. That is, it is desired that cracking reactions be significantly limited to the reaction in the reactor riser followed by a very fast separation of hydrocarbons from the catalysts in order to prevent unwanted reactions or “overcracking” of the hydrocarbon feedstocks and/or reaction products. Therefore, most modem fluidized catalytic cracking units incorporate a quick hydrocarbon/catalyst separation mechanism after the hydrocarbon/catalyst stream leaves the reactor riser. Mechanical cyclones, as discussed above, are generally the most common method utilized for making the bulk of the catalyst/oil separation in the fluidized catalytic cracking processes.
U.S. Pat. No. 5,190,650 to Tammera et al. also illustrates a common feature in short contact-time fluid catalytic reactor riser designs in that a “gap” or “disengaging zone” between the reactor riser and the primary cyclones is commonly supplied in the design of the reactor. This is also illustrated in FIG. 1 of the present application which illustrates this prior art. This gap is incorporated between the riser and the primary cyclones in “negative pressure” reactor designs, wherein the primary cyclones are operated at a lower (or negative pressure) in relation to the dilute phase of the FCC reactor. This gap can also be incorporated between the primary cyclones and secondary cyclones in “positive pressure” reactor designs, wherein the primary cyclones are operated at a higher (or positive pressure) in relation to the dilute phase of the FCC reactor. This gap or “disengaging zone” is utilized to allow the removal of hydrocarbon vapors and steam from the FCC reactor via the cyclones.
U.S. Pat. No. 4,606,814 to Haddad et al. illustrates an FCC riser/cyclone arrangement wherein the riser is attached to a cyclone without a gap. In this invention, some of the catalyst in the riser is separated prior to the primary cyclone. Additionally, due this design, some of the hydrocarbons are also entrained in the catalyst of this first separation. This leads to uncontrolled catalyst/hydrocarbon contact time and can lead to overcracking of the hydrocarbon feed materials. Additionally, a significant portion of the hydrocarbons enter the dilute phase of the reactor and must reenter the reactor riser, again leading to overcracking of the hydrocarbon feed materials.
U.S. Pat. No. 4,394,349 to Cartmell illustrates a similar arrangement to the Haddad reference wherein at least a significant portion of the catalyst and hydrocarbons in the reactor riser are allowed to be removed to the dilute phase section of the reactor. Similar to the Haddad design, this uncontrolled catalyst/hydrocarbon contact time in the dilute phase of the FCC reactor results in undesired overcracking of the hydrocarbon feed materials.
U.S. Pat. No. 5,368,721 to Terry et al. illustrates a reactor riser arrangement in which a disengaging zone is provided for in the reactor riser. However, in the Terry invention, the bottom portion of the riser is increased in cross-sectional area to decrease the “slip velocity” of the reaction stream, followed by a “slip velocity increasing means” and further a “velocity reducing means”. These elements tend to increase the resistance in the riser and provide additional hardware that can be subject to fouling. These elements also have the effect of a reduction of flow velocities in the reactor riser followed by an increase in the flow velocities in the reactor riser. These alternating velocities in the reactor riser can lead to undesirable pressure fluctuations in the reactor, riser, and/or cyclones. Additionally, the reduced overall velocity of the reaction stream in the riser can lead catalyst dropout from the riser and the loss of hydrocarbon feed into the dilute phase of the reactor through the riser disengaging zone.
U.S. Pat. No. 4,579,716 to Krambeck et al. (referred to herein as “Krambeck”) and U.S. Pat. No. 4,588,558 to Kam et al. (referred to herein as “Kam”) (both patents collectively referred to herein as the Krambeck/Kam patents) feature a disengaging zone in the FCC reactor similar to the present invention. These two patents assigned to Mobil Oil Corporation are based on similar designs and have similar deficiencies. One major deficiency with these designs is that they cannot be operated properly unless there is a very high amount of concentricity between the lower portion of the riser and the upper portion of the riser in the disengaging zone. In particular, in the Kam patent it is stated that “the two portions of the riser must be aligned so the maximum eccentricity is 10%” (see Kam, U.S. Pat. No. 4,588,558, column 6, lines 9-10). As explained in Kam, if the two portions of the riser are not maintained in a “substantially concentric relationship to each other”, significant backmixing of catalyst and hydrocarbons will occur (see Kam, U.S. Pat. No. 4,588,558, column 6, lines 2-8). The problem with the Krambeck/Kam designs is that due to the high temperatures in the FCC reactors (typically in the range of about 950 to 1250° F.) and resulting significant thermal expansion of the reactor components, it is difficult, if not impossible, to maintain this degree of concentricity under operating conditions. In order to address this problem, the Krambeck/Kam patents attempted three ways in which to address this problem.
The first, and simplest, manner in which this concentricity problem was addressed by the Krambeck/Kam designs is by using “mechanical spacers” to maintain the concentricity of the upper and lower risers. This is illustrated in U.S. Pat. No. 4,588,558 to Kam et al. wherein “three or more spacers 45 are provided between the two sections of the riser to maintain the two sections concentric and separated from each other by an equal distance” (see Kam, U.S. Pat. No. 4,588,558, column 9, lines 37-40; and element 45 in FIG. 1). However, there are multiple problems with this design. The first resulting problem is that these spacers constrict the flow area of the annulus resulting in higher pressure drop and uneven flow patterns for the entering vent gas from the dilute phase of the reactor. The second resulting problem is that this creates erosive material conditions between the spacers and the riser. This is especially problematic for design as it is desired that the riser internal to be covered with an erosion resistant refractory or coating to protect from wear induced by the high velocity catalyst moving through the riser as well as entrained catalyst entering the annulus from the FCC reactor dilute phase. This spacer design leaves an exposed area for catalyst and mechanical contact wear thereby reducing the mechanical reliability of the design. The third resulting problem, and possibly most significant drawback of the spacers design is that the upper and lower risers transfer mechanical forces between each other via the spacers, which due to the induced thermal expansion of these components being in different directions relative to one another, can create significant unwanted mechanical forces emanating from the lower internal riser unto the upper internal riser as well as the attached cyclone configuration. This results in significant mechanical overdesign of related components as well as reduced reliability of the unit. What is needed in the art is a design that does not rely on spacers or mechanical contact points between the upper and lower riser assemblies.
The second manner, in which this concentricity problem was addressed by the Krambeck/Kam designs is by the installation of “trickle valves” in the conduits between the riser and the primary cyclones, as well as in the conduits between the primary cyclones and secondary cyclones. These can be see in U.S. Pat. No. 4,579,716 to Krambeck et al. in FIG. 1 as elements 27, 27A, 41, and 41A, and in U.S. Pat. No. 4,588,558 to Kam et al. as elements 22 and 38. It is believed that the use of these trickle valves was to compensate for erratic flows and pressure surges that can accompany the use of the Krambeck/Kam designs. The resulting problem with this design is that it results in reinstituting one of the main problems that is trying to be eliminated by installing the disengaging zone in the riser. The problem is that in the present art, the opening or gaps are installed in the conduits in the same general locations as the trickle valves in the Krambeck/Kam designs and are both prone to a large amount of backmixing of catalysts (solids) and hydrocarbons (vapors) as well as excessive coking. What is needed in the art is a design that does not rely on secondary openings in the cyclone conduits for the entry or expulsion of catalysts and hydrocarbons to and from the dilute phase section of the FCC reactor.
Lastly, a third manner in which this concentricity problem was addressed by the Krambeck/Kam designs relies on openings (or “gaps”) in the conduit between the riser and the primary cyclones. This is shown in U.S. Pat. No. 4,579,716 to Krambeck et al. in FIG. 1 as elements 22 and 38. This design again imposes the same problems which are desired to be eliminated and discussed in the paragraph above.
It should also be noted that due, probably at least in part, to the drawbacks in the Krambeck/Kam designs identified and discussed above, a design incorporating a disengaging zone in the reactor riser as shown in the Krambeck/Kam references was never commercially implemented by Mobil Oil Corporation.
What is required in the industry is an improved FCC reactor design which can ensure that substantially all of the hydrocarbon feed and catalyst in the reactor riser is delivered to the cyclone system for effective and controlled separation and does not impart significant feed fluctuations or hardware in the reactor riser, while improving the mechanical design of the overall system.